Barometers Realm

Before us is a Fine Early Vidie Aneroid Barometer by Lucien Vidie, the inventor of the aneroid barometer principle itself. The instrument was made around 1860, at a time when the aneroid mechanism was still undergoing active engineering development and retained many characteristic features of Vidie’s first experimental and semi-serial mechanisms. Although the mechanism was devised by Lucien Vidie, the instrument was sold under the name of the renowned Parisian firm Lerebours & Secretan, one of the leading manufacturers and retailers of scientific instruments in nineteenth-century France. It was companies of this kind that helped introduce the aneroid into scientific and engineering circles. In Britain, a similar role was played by the celebrated clockmaker Edward John Dent, who was among the first to recognise the potential of Vidie’s invention and to begin the commercial distribution of his instruments on the English market.

The case of the instrument is made in the deep barrel-shaped form characteristic of early Vidie aneroids and is fashioned from tombac alloy, a high-copper brass alloy with a warm golden tone. At the front, the case is closed by a thin decorative bezel with a fine ornamental pattern, holding a flat mineral glass. Mounted at the centre of the glass is a brass index pointer with a knurled setting knob, allowing the observer to set a reference mark manually in order to follow changes in pressure. At the top of the case is a suspension ring, enabling the instrument to be used as a wall barometer.

The dial is made of silvered brass and carries an engraved barometric scale in centimetres of mercury, ranging from 72 to 80 cm Hg. Along the outer edge of the scale is a detailed French weather indication — Tempête, Grande Pluie, P. Vent, Variable, Beau T., Beau Fixe, Très Sec — inherited by aneroid barometer dials from the register plates of earlier mercury barometers.

Particularly noteworthy is the inscription “Baromètre Anéroïde”, arranged in an arc within the inner part of the scale. It is executed in an elegant calligraphic script with pronounced cursive elements and decorative flourishes, sharply contrasting with the stricter, more utilitarian weather terms of the outer scale.

The lower part of the dial is occupied by an arched mercury thermometer with a centigrade scale engraved directly onto the dial surface.

Around the entire outer edge of the dial is a brass reflector ring, serving both a structural and an optical function. On the one hand, it secures the dial within the case; on the other, it provides an even reflection of light, visually emphasising the hands and facilitating the reading of the instrument under side illumination.

The principal value of this instrument, however, lies not so much in its external appearance as in its exceptionally early Vidie-system movement.

This is a movement from the period when Lucien Vidie had already abandoned the external coil spring characteristic of the earliest aneroids of the 1840s and had adopted the famous C-shaped leaf spring, the so-called swan-neck spring. This construction later became conventional for subsequent aneroid barometers and, in modified form, remains in use even today.

At the heart of the mechanism is an evacuated corrugated capsule made of nickel silver, rigidly secured by its lower diaphragm to a massive steel mounting plate. Under atmospheric pressure, the thin walls of the capsule tend to deform, and the entire subsequent kinematic system serves to convert these microscopic movements into the motion of the pointer.

Above the capsule is a massive cast-iron support bridge, a heavy cast load-bearing element mounted on two vertical supports. Unlike later lightweight brass bridges and stamped frames, this mechanism employs an exceptionally rigid cast structure, minimising parasitic deformation and ensuring the stable geometry of the entire mechanism.

Each support ends in a small contact pad, or “foot”, through which the bridge rests on the mounting plate. The main body of each support is deliberately raised above the base, so that actual contact occurs only at limited points. This arrangement reduces internal stresses within the structure and prevents distortion of the mechanism.

The left support is fitted with a vertical adjusting screw passing through its central section and bearing against the main platform. By turning this screw, the entire bridge can microscopically alter its position relative to the base. Since the lower end of the C-shaped spring and the associated kinematic system are attached to this bridge, a change in the bridge position simultaneously alters the pre-tension of the whole system and serves as a means of fine calibration.

The right support is constructed differently: its lower part is provided with a thickened guiding projection, engaging loosely with a slotted opening in the main plate. This element does not rigidly fix the bridge, but acts as a positioning guide, allowing the mechanism during assembly to settle into a position with minimal internal stress and without misalignment of the linkages.

As noted above, one end of the C-shaped spring is rigidly fixed to the cast-iron bridge, while its opposite edge is connected along its length to a massive horizontal cylindrical rod, the main force-transmitting element of the mechanism. This horizontal cylindrical rod functions as a kind of transverse transmitting bar, an intermediate element converting the deformation of the spring into movement within the subsequent kinematic system. From it, at a right angle, extends a thinner main lever, connected to the principal arbor of the mechanism.

The connection between this system and the upper diaphragm of the aneroid capsule is particularly elegant. It is achieved by means of a simple but highly effective steel hook-shaped element, resembling the figure “7”. The lower part of this hook is fixed to the upper surface of the capsule by a transverse pin, while its upper bent end freely embraces the horizontal cylindrical transmitting bar. This creates a semi-articulated connection with a certain degree of freedom. Such an arrangement allows microscopic angular deviations without jamming, compensates for slight misalignment during assembly, reduces parasitic stresses, and enables the sensitive diaphragm to operate almost without lateral loading.

The principal transmitting assembly of the mechanism consists of a two-armed rocking bell-crank lever made of brass, mounted between two supports. This element corresponds to what is described in early French sources as the chariot à ressort, a distinctive balancing transmission unit of the Vidie system. The lever rotates freely about its own axis and acts as an intermediate converter between the elastic system of the aneroid capsule and the pointer mechanism of the instrument. On the left side, its support is provided by a pointed steel screw, the conical end of which enters a small centring recess. This pivot-bearing system provides an extremely small contact area and therefore minimal friction during the rocking motion of the lever. The opposite side is held in its support by a guiding pin.

Motion is transmitted to bell-crank lever through the bielle oblique, an oblique connecting link joining the main lever to the horizontal arm of the rocking bell-crank lever. It is this inclined linkage that converts the vertical micro-deformations of the aneroid system into the angular rocking motion of the principal transmitting unit.

Attached to the short vertical arm of the bell-crank lever, through an extension piece, is a fine fusee chain, which transmits motion to the pointer arbor. The use of a chain instead of a rigid rod eliminates lateral loading, compensates for microscopic misalignments, and allows extremely small movements to be transmitted with almost no parasitic friction.

The massive knurled cylindrical element located on one arm of the lever, despite its resemblance to an adjusting knob, most probably functions as a balancing weight or inertial counterweight. Its purpose is to stabilise the neutral position of the lever, reduce sensitivity to vibration, and suppress parasitic oscillations in the system as a whole.

The final indicating assembly, the pointer arbor, is formed as a rotating pointer drum, converting the small linear movements of the mechanism into the angular movement of the pointer over the scale.

The construction of the pointer arbor is especially noteworthy. The pointer drum is not mounted directly on a single solid arbor, but through an intermediate cylindrical clamping sleeve surrounding the arbor. This sleeve forms an independent driving unit, connected to the pointer arbor by a transverse locking screw. The screw passes simultaneously through the sleeve and the arbor itself, forming a rigid yet adjustable connection. This construction allowed the maker, during assembly, to set independently the pre-tension of the hairspring, the position of the pointer, the tension of the fusee chain, and the neutral position of the entire kinematic system.

Motion is transmitted by the fusee chain, which winds onto a special drum. Unlike later simplified constructions, the chain does not wind chaotically, but is laid into specially cut guiding grooves, forming a strictly sequential multi-layer winding. This arrangement preserves a constant effective winding radius, reduces parasitic losses, prevents the links from crossing or rubbing against one another, and improves the linearity of the transmission.

Mounted on the same arbor is a spiral hairspring, a fine flat return spring concentrically arranged around the axis of the drum. The inner end of the hairspring is fixed directly to the rotating arbor, while its outer end is secured to a stationary part of the mechanism. As the pointer drum rotates, the hairspring is wound and creates an opposing elastic torque. It is this spring that maintains constant tension in the fusee chain, eliminates backlash, stabilises the position of the pointer, and returns the system to its initial position as atmospheric pressure changes.

The entire pointer assembly is held by a characteristic cantilever-type brass bridge, shaped like an inverted angular bracket. The bridge extends from the support of the main arbor and holds the pointer drum between two plates, ensuring stable positioning of the axis while keeping the construction as light as possible.

Overall, this barometer represents an exceptionally important and rare example of the early engineering evolution of Vidie’s aneroid mechanism, from a period when the construction still retained an almost experimental complexity and remarkable mechanical refinement. Later simplifications for mass production are still absent here, and every element of the mechanism demonstrates the search for the most effective way to transmit the almost imperceptible deformations of the evacuated capsule into the smooth and precise motion of the pointer. For this reason, early Vidie aneroids of this type possess not only historical significance but also exceptional engineering value, standing among the most delicate mechanical measuring systems of the mid-nineteenth century.